sixty adj., n.

one more than five; twice three; the number which is one more than five; a symbol representing this number;" Old English siex, six, seox, sex, from Proto-Germanic *seks (source also of Old Saxon and Danish seks, Old Norse, Swedish, and Old Frisian sex, Middle Dutch sesse, Dutch zes, Old High German sehs, German sechs, Gothic saihs), from PIE *s(w)eks (source also of Sanskrit sas, Avestan kshvash, Persian shash, Greek hex, Latin sex, Old Church Slavonic sesti, Polish sześć, Russian shesti, Lithuanian šeši, Old Irish se, Welsh chwech).

As "playing card with six spots or pips" by 1590s. Six-footer "person measuring six feet or more" is by 1828. Six-shooter, usually a revolver with six chambers (firing six shots in succession), is attested from 1842; six-pack of beverage containers is from 1952, of abdominal muscles by 1995.

[Married] In Dana, [Mass.], on the 30th ult. by Ephraim Whipple, Esq. Mr. Jason W. Williams, M. D. to Miss Malinda Chamlin, of Dana,—a couple of six footers. Printers in New York, Ohio, Alabama, Vermont, Connecticut, Rhode-Island, New Hampshire and Maine, are requested to publish the same marriage, that his brothers and sisters may have due notice thereof and rejoice accordingly. [Burlington, Vt., Weekly Free Press, Feb. 8, 1828]

Six of one and half-a-dozen of the other "little difference" is recorded from 1833. Six-figure (adj.) in reference to hundreds of thousands (of dollars, etc.) is from 1840. Six feet under "dead" is from 1942.

The phrase at sixes and sevens originally was "hazarding all one's chances," first in Chaucer, perhaps from dicing (the original form was on six and seven); it could be a corruption of at cinque and sice "exposed to great risk" (1530s), literally "at five and six," using the French names (which were common in Middle English) for the highest numbers on the dice. The meaning "at odds, in disagreement or confusion" in the exact phrase is from 1785; in Middle English the phrase set at (or on) six and seven meant "play havoc, create an uproar."

suffix representing "ten" in cardinal numbers that are multiples of 10 (sixty, seventy, etc.), from Old English -tig, from a Germanic root (cognates: Old Saxon, Dutch -tig, Old Frisian -tich, Old Norse -tigr, Old High German -zug, German -zig) that existed as a distinct word in Gothic (tigjus) and Old Norse (tigir) meaning "tens, decades." Compare tithe (n.).

English, like many other Germanic languages, retains traces of a base-12 number system. The most obvious instance is eleven and twelve which ought to be the first two numbers of the "teens" series. Their Old English forms, enleofan and twel(eo)f(an), are more transparent: "leave one" and "leave two."

Old English also had hund endleofantig for "110" and hund twelftig for "120." One hundred was hund teantig. The -tig formation ran through 12 cycles, and could have bequeathed us numbers *eleventy ("110") and *twelfty ("120") had it endured, but already during the Anglo-Saxon period it was being obscured.

Old Norse used hundrað for "120" and þusend for "1,200." Tvauhundrað was "240" and þriuhundrað was "360." Older Germanic legal texts distinguished a "common hundred" (100) from a "great hundred" (120). This duodecimal system is "perhaps due to contact with Babylonia" [Lass, "Old English"].